We consider how experimental data for nucleation rates can be analyzed if it is assumed that classical nucleation theory is a reasonable description of the nucleation. In particular, we consider in detail experimental results for the nucleation of the protein lysozyme. Classical nucleation theory makes predictions for the functional form of the nucleation rate; physically reasonable bounds can be placed on the parameters of this functional form. The experimental data for lysozyme implies values for a parameter (the preexponential factor) that are outside the physically reasonable bounds for homogeneous nucleation but are consistent with heterogeneous nucleation. Also, the functional form of the rate suggests that there is a distribution of barrier heights. Such a distribution is likely for heterogeneous nucleation but not possible for homogeneous nucleation. Modeling experimental data with classical nucleation theory is a generally applicable way of distinguishing between homogeneous and heterogeneous nucleation.